This invention concerns the improvement of pigment pastes, for use in aqueous coating compositions containing a film-forming latex polymer and a pigment, by incorporating into the paste a novel dispersant polymer of great versatility. The dispersant polymer is an effective dispersant for a wide range of pigments and useful in making a pigment paste therefrom; it is also an effective dispersant for the pigment in the presence of the other components of many coating compositions. For example, the dispersant does not appreciably affect the corrosion-resistance properties of a reactive pigment paint in which it is incorporated.
The use of low molecular weight polyelectrolytes as dispersing agents for pigments in water based coatings is known. U.S. Pat. No. 3,980,602 discloses dispersants containing 5to 25% by weight of acrylic or methacrylic acid in polymers based on alkyl acrylates and certain alkyl methacrylates; styrene may be substituted for the latter. These polymers having weight average molecular weights in the range 500 to 30,000. Dispersants based on copolymers of olefins and maleic ahydride are disclosed in U.S. Pat. No. 2,930,775. Formulations containing these dispersants may often exhibit poor stability in the presence of divalent cations. U.S. Pat. No. 3,840,487 discloses acrylic polymer dispersants for high gloss paints the dispersants being composed of 40 to 95 mole percent unsaturated monocarboxylic acid, such as acrylic or methacrylic acid, and 5 to 60 mole percent unsaturated carboxylic acid esters, such as the esters of these acids. Average molecular weights below 75,000 are mentioned. British Pat. No. 985,276 discloses a pigment dispersant comprising acrylic acid, methacrylic acid, an ester of methacrylic acid and a monohydric alcohol and an ester of methacrylic acid and a polyhydric alcohol. The polymerization are in a aqueous medium and produce an emulsion copolymer. Canadian Pat. No. 655,884 discloses dispersants which are copolymers of methacrylic acid and esters of acrylic and methacrylic acids made by a process employing high levels of persulfate initiators such that sulfate groups, believed to be terminal on the polymer chain, are responsible for the enhanced dispersing activity recited for the polymers.
A pigment paste is a pigment concentrate made to permit substantial reduction with solvents, water or vehicle (paint components other than solid pigment) in the preparation of a paint. The pigment paste, also known as pigment dispersion, comprises pigment, dispersant, defoamer, water and/or a glycol and is used to formulate a latex paint. One common way of making latex paints is to charge the paste components to a dispersion mill in which the pigment is dispersed in the aqueous or glycol system and then "letdown" with the remainder of the paint constituents, normally comprising latex, thickener, water, and other specialized additives, often in the small mill running at lower speeds.
Pigments are normally supplied in the fine particle size necessary for use to form thin coating films. For use in paint, the particles must be thoroughly wetted and pigment agglomerates broken up in a high-speed disperser, sand mill or other dispersing equipment. Once a good pigment dispersion is obtained, it must be maintained through the process of paint manufacture and storage. The role of the dispersant is to aid in the dispersion of pigment and thus to develop the full value of hiding and color to be contributed by the pigment. The dispersant also aids in fluidizing the pigment slurry for ease of milling, increases the stability of the pigment suspension in water or glycol, or a mixture thereof, and finally the stability of the completed paint. In considering dispersants and pigment pastes made therefrom, many paint formulations present few difficulties, but if a dispersant is to be effective in a wide range of formulations it must perform well when used in the difficult-to-formulate paint systems. Often the performance of a dispersant and the pigment paste made therefrom is limited by there being a comparatively low top limit to the amount of dispersant which may be used without producing deleterious effects. Another factor which limits utility of many dispersants is incompatibility with the so-called reactive pigments, particularly those used in corrosion inhibiting paint formulations. Thus, such a paint formulation serves as an excellent screening system to demonstrate the versatility of dispersants for use in pigment pastes. The dispersant of the instant invention permits the formulation of pigment pastes containing reactive and corrosion inhibiting pigments which may be used to prepare paints very stable in the wet state and which deposit films imparting a high degree of corrosion resistance when dry. The dispersant of the instant invention permits the corrosion resistant property to be maintained even when used at high concentration in the paste. Thus, pastes made with the dispersant of this invention can be used to make paints which achieve a balance of high stability and high corrosion resistance as well as the many other desiderata of high quality paints.